In spring of 1942, US cryptanalysts had great success reading the Japanese naval code but so far in the war, the results have been mixed at best. Still, the cryptanalysts continued to decode Japanese radio transmissions and they knew the next Japanese target was “OBJ A-F”. With only two remaining American aircraft carriers operational, the USS Enterprise and the USS Saratoga (the USS Yorktown was still limping back from the debacle in the Coral Sea) Admiral Nimitz had to know where OBJ A-F was so he could ambush the six fleet carriers he expected to participate in the next Japanese operation. The list of possible targets included another raid on Ceylon (Sri Lanka), American Samoa, Hawaii, the Aleutian Islands, and several others.
Cmdr Joseph Rochefort, the senior intelligence officer of the Pacific fleet, thought it might be Midway Island and used a radio deception to confirm. He sent a cable to the garrison to tell them to send a radio transmission in the clear stating that their freshwater distillation system was broken and they needed freshwater as soon as possible. The confused radioman at Midway, who walked past the fully functioning system all the time, initially questioned the order but sent the message anyway. On 15 May 1942, Cmdr Rochefort’s cryptanalysts decoded a Japanese message saying that OBJ A-F requests freshwater. Nimitz immediately began preparing his ambush of the Japanese carriers as they approached Objective A-F, Midway Island.